Strategies used to stabilize the elbow joint challenged by inverted pendulum loading

The stiffness of activated muscles may stabilize a loaded joint by preventi ng perturbations from causing large displacements and injuring the joint. H ere the elbow muscle recruitment patterns were compared with the forearm lo aded vertically (a potentially unstable inverted pendulum configuration) an d with horizontal loading. Eighteen healthy subjects were studied with the forearm vertical and supina ted and the elbow flexed approximately 90 degrees. In the first experiment EMG electrodes recorded activity of biceps, triceps, and brachioradialis mu scles for joint torques produced (a) by voluntarily exerting a horizontal f orce isometrically (b) by voluntarily flexing and extending the elbow while the forearm was loaded vertically with 135 N. The relationship between the EMG and the torque generated was quantified by the linear regression slope and zero-torque intercept. In a second experiment a vertical load increasi ng linearly with time up to 300 N was applied. In experiment 1 the EMG-torque relationships for biceps and triceps had an intercept about 10% of maximum voluntary effort greater with the vertical c ompared to the horizontal force, the inverse was found for Brachioradialis, but the EMG-torque slopes for both agonist and antagonistic muscles were n ot different. In experiment 2 there were 29 trials with minimal elbow displ acement and all the three muscles activated on the order of 11% of maximum activation to stabilize the elbow; 19 trials had small elbow extension and 14 trials small flexion requiring altered muscle forces for equilibrium; 7 trials ended in large unstable displacement or early termination of the tes t. An analysis indicate that the observed levels of muscle activation would only provide stability if the muscles' short-range stiffness was at the hi gh end of the published range, hence the elbow was marginally stable. The s tability analysis also indicated that the small elbow extension increased s tability and flexion decreased stability. (C) 2000 Elsevier Science Ltd. Al l rights reserved.